Mode 1. Period = 0.99 sec Mode 3. Period = 0.89 sec
"Almost all practical structures are more complex than the 'single degree of freedom (SDOF) systems discussed so far. However, many structures can be idealized as SDOF systems. The response of many buildings is dominated by their fundamental sway mode in each direction. Irvine, in his excellent textbook 'Structural dynamics for the practising engineer' shows how many systems encountered in engineering practice can be treated in this way."
Edmund Booth, from 'Analysis for earthquake effects' in 'Concrete structures in earthquake regions'